The present invention relates to terminal bushings and more particularly to terminal bushings used with electrical equipment containing dielectric gases or liquids.
Terminal bushings are employed in high voltage electrical equipment to prevent arcing, or flashover, between the grounded shell or tank surrounding the equipment and the line, or conductor, projecting therefrom. A typical bushing consists of a skirted weathershed which surrounds the conductor. The electric potential between the conductor and equipment is commonly sufficient to cause flashover through the air between the conductor and equipment in the absence of a bushing. Bushings are made from electrically insulative materials, such as ceramics or epoxy resins. However, at the high voltages with which the bushings are used, the possibility of flashover exists even with such insulative materials.
Electric flashover may occur along several distinct paths, or along a combination of these paths. One potential flashover path is along the outer skin of the bushing from its apex, adjacent the protruding conductor, to the tank housing the electrical equipment. A second path exists along the inner skin of the bushing from its base, adjacent the conductor, to the tank. The conductor may flashover through the wall of the bushing to the tank. In bushings having internal cavities, a flashover may occur from the conductor across the cavity and onto the tank.
To prevent flashover along the skin of the bushing, the linear distance of the bushing skin from the tank to the apex of the bushing adjacent the conductor, called the string distance, is increased by employing a series of skirts. The outer surface, or skin of the bushing, has a relatively constant dielectric strength, or resistance to current flow, per given increment of length. Therefore, the skirts increase this length thereby increasing the overall dielectric strength of the bushing. The skirts extend from the exterior of the bushing in the form of circular fins. To minimize the conductivity of the bushing within the tank, and to likewise minimize its ,length therein, the tank is commonly filled with a dielectric medium such as SF.sub.6 gas. The bushing material will have a lower surface conductivity in a dielectric gas than in air, and therefore the string distance from the protrusion of the conductor through the base of the bushing to the tank can be substantially less than the string distance from the apex of the bushing to the tank.
Generally, prior art bushings have two internal constructions: a solid core surrounding the conductor, or a hollow core having seals adjacent the ends of the bushing to prevent the escape of the dielectric medium. The prior art includes bushings made from ceramic as well as resinous materials.
Solid core bushings have several disadvantages. First and foremost, they take little advantage of the dielectric medium in the tank, and therefore must surround the conductor a substantial distance into the tank to prevent flashover. Likewise, they tend to be bulky and heavy, which requires a large amount of raw material.
Prior art hollow bushings suffer the disadvantage of having several leak paths for escape of the dielectric gas. For example, U.S. Pat. No. 4,431,859 to Kishida discloses a hollow bushing. It should be appreciated that the bushing of Kishida requires the use of a seal at its apex to prevent the escape of gas, as well as a seal adjacent the base of the bushing near the tank.
U.S. Pat. No. 3,178,505 to Van Sickle discloses a terminal bushing for gas-filled transformer applications. The disclosed device includes an inverted conical insulator section which protrudes within the bushing. However, the entire bushing is hollow and filled with the insulating gas. As in Kishida, the bushing requires seals at each end of the bushing.
Other objects and advantages of the present invention will appear from the following description.